In the manufacture of ceramic parts using dry milling, powder ingredients are first admixed and dry milled to more uniformly mix the ingredients and to provide a better particle size and particle size distribution by comminution through the milling action. In addition, if attrition or wear from the milling media is a desired or required part of the chemistry of the powder composition, the milling action is used to create the proper chemical proportions of the mix. The dry milled powder mixture can then be directly subjected to cold compaction, gas chemical treatment, and consolidation to produce the desired product. Dry milling is advantageous, as opposed to other types of milling, because it promotes a broader particle size distribution which, in turn, provides much better packing density for improved hot pressing. However, if the dry milling is not complete, that is, if the powder ingredients are not reduced to a desired maximum particle size and desired homogeneity, then incomplete or unreacted ingredients may result from subsequent gas chemical treatment, resulting in a hot pressed product with flaws and hot pressed products with variable physical properties.
In the case of the manufacture of a silicon nitride cutting tool material, flaws are associated with inconsistent dry milling techniques and are attributed to homogeneity differences and to subsequent processing treatments. Incomplete dry milling often is the result of caking of the powder mixture along the sides of the milld due to the hygroscopic nature of silicon. The resulting homogeneity differences include a variable surface area mixture which limits effective oxygen control and concentration gradients of the ingredients which causes strength variation in the final hot pressed product. In addition, processing flaws also originate from unreacted silicon after nitrogen gas treatment of silicon powder. The free silicon is due to large particles that are not fully nitrided. Free silicon eventually combines with impurities in the silicon powder either during nitriding or during hot pressing to generate a silicide, such as iron silicide. The silicide forms a liquid which dissolves silicon nitride during hot pressing and, when cooled, crystallization results in large beta Si.sub.3 N.sub.4 particles adjacent the silicide along with porosity. These are defects in the ceramic which may pull out or degrade mechanically or chemically during use of the ceramic, particularly as a cutting tool.
Heretofore, prior art methods have incorporated wet milling aids in a high amount of 5% or more by weight, such as stearic acid, zinc stearate, oleic acid, and carbowax, each of which have a high residual carbon content in the final product which can cause contamination. The importance of carbon control and the effects of carbon on hot pressed silicon nitride is described in the article by H. Knoch and G. E. Gazza, "Effect of Carbon Impurity on the Thermal Degradation of an Si.sub.3 N.sub.4 --Y.sub.2 O.sub.3 Ceramic", Journal of The American Ceramic Society, 62 (11-12) 634-635 (1979). Although wet particle milling appears to avoid the caking problem associated with dry milling, such wet milling undesirably provides carbon contamination of the mixture because of the necessity for large amounts of liquid resins (containing carbon) to facilitate the wet milling (see British Pat. No. 966945). Moreover, wet milling inherently provides too steep a particle distribution for good packing density and thus eliminates one of the significant advantages of dry milling. Wet milling must be distinguished from processes where liquid resins are added to the powder mixture to provide for molding of the mixture into a body which is then nitrided and sintered (for the latter, see U.S. Pat. Nos. 4,209,477; 4,346,147; 4,351,787; 4,354,990; 4,243,621; 4,221,596; and German Pat. No. 1,949,587). Any inherent lubricating quality of such liquid resins is, of course, unemployed in such prior art usage.
What is needed is a method for dry milling the starting powder ingredients consistently to ensure that the particles of the milled dry mixture are not above a critical particle size, such as 35 microns. This requires preventing the packing of the mixture along the sides of the milling chamber and thus ensuring a constant milling action. Such method should not incorporate additive materials that will provide for undesirable side reactions, such as carbon contamination. Additiionally, such method should optimally reduce the milling time, should improve the green strength of the milled mixture when compacted, and should minimize, if not totally eliminate, flaws associated with unmilled powder in the final ceramic product. It should also provide for a reproducible milling action from batch to batch, i.e., reproducible homogeneity.